Efficient vehicle dynamics are achievable in vehicles having all-wheel drive via a controlled distribution of the drive power to the front and rear axles. A transfer case (VG) is used as an actuator for power distribution. The main part of this transfer case is a multidisk clutch (MSK) which transfers the torque to the power take-off side of the vehicle as a function of the pressing force exerted on its lamellas. The mechanical construction of the transfer case allows the precision distribution required within the specification, exclusively by exerting a force on the actuating mechanism. This actuating force is generated, for example, using an eccentric disk and a pantograph mechanism, in general by a geared or positioning motor (GM), in particular by a DC motor having a worm gear. FIG. 1 shows the actuating chain of the transfer case and its components: DC motor (101), worm gear (102), eccentric disk (103), pantograph (104), and multidisk clutch (105).
For cost reasons, force or torque sensors are frequently omitted in the design of the geared motor's control. Instead, the actuating characteristic of the transfer case is saved in the control unit (SG) in the form of a torque-actuator travel characteristic curve (201) (FIG. 2), whereby the actuating intervention is attributed to a positioning of the eccentric disk, i.e., to a position regulation of the geared motor. The central point of the characteristic curve is the engagement point (202) also known as the kiss point. This is the point at which the multidisk clutch begins to transfer torque. The multidisk clutch setting as a function of the length of operation causes an angular shift in the characteristic curve stored in the control unit.
A calibration procedure may be used to detect the shift in the point of engagement. The speed-regulated geared motor is used here as a sensor to reconstruct the point of engagement. The eccentric disk is rotated by the positioning motor at a constant speed against the actuator load torque generated by the clutch. If the motor current is then recorded using measuring technology (FIG. 3), it may be averaged at three characteristic angular positions of the eccentric disk. Two straight lines (301, 302) may then be constructed using the three current/angle points, the point of intersection of which would represent the point of engagement. Straight line (301) is assumed here to be a horizontal line.
However, the motor current represents the actuating torque only for a constant and precisely known transmission efficiency. For the positioning motors and actuating mechanisms normally used, the efficiency varies not only as a function of the individual component and the service life, but also, for example, as a function of the worm wheel angle (which tooth of the worm wheel is engaged). In the case of speed-regulated operation, an efficiency which varies over the worm wheel position results in a current excitation, i.e., in a local distortion of the current characteristic curve, FIG. 3. Such a distortion, if located in the range of the averaging points, results in erroneous determination of the point of engagement.